The photoreceptor outer segment is an elaborate primary cilium. The delivery of opsin into and along the cilium is critical for photoreceptor cell function and viability. The overall goal of this study is to understand the cellular mechanisms involved in these delivery processes. A combination of cell biological methods, including imaging of protein movements in the cilium of live mouse photoreceptor cells, will be used. Studies will address questions about opsin entry into and transport along the cilium, and the membrane diffusion barrier of the photoreceptor cilium. The proposed research incorporates technical innovation, including the measurements of real-time movements of opsin along the cilium, using a ciliated epithelial cell line and mouse rod photoreceptor cells. It will test novel concepts of opsin trafficking and the organization of the photoreceptor cilium.